1. Field of the Invention
The present invention relates to a semiconductor device having a circuit structured by a thin film transistor (hereafter referred to as a TFT) on a substrate having an insulating surface, and to a method of manufacture thereof. In particular, the present invention provides an electro-optical device, typically a liquid crystal display device or an EL display device in which a pixel portion and a driver circuit are formed on the same substrate, and provides a technique of suitably utilizing this type of electro-optical device loaded into electronic equipment. Note that, throughout this specification, the term semiconductor device indicates a general device functioning by utilizing semiconductor characteristics, and the above electro-optical device, and equipment in which the electro-optical device is loaded, are included in the category of semiconductor device.
2. Description of the Related Art
A display device structured by a pixel portion in which active elements are arranged is referred to as an active matrix display device, and devices such as a liquid crystal display device and an electroluminescence (hereafter referred to as EL) display device have been developed. An insulating gate type transistor is used in an active element, and a TFT is used ideally. A semiconductor film is formed on a substrate such as glass by a method such as a vapor phase growth method, and, using the semiconductor film, regions such as a channel forming region, a source region, and a drain regions are formed for the TFT. Ideally a material having silicon as its main constituent, such as silicon or silicon germanium, is used in the semiconductor film. Semiconductor films can be classified into amorphous semiconductor films, typically amorphous silicon, and into crystalline semiconductor films, typically polycrystalline silicon, in accordance with their method of manufacture. In addition, techniques for structuring a pixel portion by insulating gate type transistors formed on a single crystal silicon substrate have been developed in recent years.
It is nearly impossible to obtain an electric field effect mobility equal to or greater than 10 cm2/V·sec in a TFT in which an active layer is formed by an amorphous semiconductor (typically amorphous silicon) film due to electrical solid state factors such as the amorphous crystal structure. Therefore, even though they can be used as switching elements for driving a liquid crystal in the pixel portion in an active matrix type liquid crystal display device (switching elements formed by TFTs are hereafter referred to as pixel TFTs), it is impossible to use them to form a driver circuit for performing image display. Consequently, a driver IC is implemented by using a technique such as TAB (tape automated bonding) or COG (chip on glass).
On the other hand, with a TFT having a semiconductor film, typically a crystalline silicon or a polycrystalline silicon, containing a crystalline structure (hereafter referred to as a crystalline semiconductor) as an active layer, a high electric field effect mobility can be obtained, and therefore this type of TFT can form all types of functional circuit and can perform driving. It therefore becomes possible to realize a pixel TFT and, on the same substrate, circuits such as a shift register circuit, a level shifter circuit, a buffer circuit, and a sampling circuit in a driver circuit. The driver circuit is formed by CMOS circuits, composed of n-channel TFTs and p-channel TFTs, as basic units. Techniques of implementing this type of driver circuit are fundamental, and in order to promote making lower weight and thinner liquid crystal display devices, it is thought that a TFT having a crystalline semiconductor layer as an active layer, in which it is possible to form the driver circuit, in addition to the pixel portion, on the same substrate, is suitable.
Forming the active layer by a crystalline semiconductor layer is superior when comparing TFT characteristics, but there are problems in that the manufacturing process becomes complex, and the number of process steps increases, in order to manufacture a TFT corresponding to each circuit in addition to the pixel TFT. It is clear that the increase in the number of process steps is a cause of increased manufacturing costs, and that it leads to a drop in the manufactured yield.
The operating conditions of the pixel TFTs and driver circuit TFTs are not necessarily the same, and therefore the required characteristics of the TFTs also differ greatly. A pixel TFT formed by an n-channel TFT is a switching element which drives a liquid crystal by applying a voltage. The liquid crystal is driven by an AC current therefore a method referred to as frame inversion drive is employed. The pixel TFT is required to have a sufficiently low Off current (the drain current flowing when the TFT is in off operation) in order to maintain an electric charge which has accumulated in a liquid crystal layer for the duration of one frame period. On the other hand, a high driver voltage is applied to the driver circuits, such as a buffer circuit, and therefore a high voltage resistance is required so that the circuit is not damaged due to the high voltage application. Further, in order to increase the electric current driver performance, it is necessary to secure a sufficient value of the On current (the drain current flowing when the TFT is in on operation).
An LDD (lightly doped drain) structure is known as a TFT structure for reducing the value of the Off current. This structure is one in which a region having a low concentration of an added impurity element is formed between a channel forming region and a source region or drain region having a high concentration of an added impurity element, and the low concentration region is referred to as an LDD region. Further, there is a GOLD (gate-drain overlapped LDD) structure, in which the LDD region is arranged so as to overlap a gate electrode through a gate insulating film, as a means of preventing degradation of the On current value due to a hot carrier. It is known that using this type of structure is effective in preventing deterioration phenomena by relieving the high electric field near a drain and protecting against hot carrier injection.
However, the bias state of the pixel TFT and driver circuit TFTs such as those of a shift register circuit and a buffer circuit are not necessarily the same. For example, a large inverse bias is applied to the gate in the pixel TFT (a negative voltage for an n-channel TFT), but driver circuit TFTs basically will not operate in an inverse bias state. Further, the GOLD structure is effective in protecting against degradation of the On current value, but the value of the Off current becomes large by simply overlapping with the gate electrode. On the other hand, although the Off current value control effect is high for a normal LDD structure, it has low effectiveness in relieving the electric field in the vicinity of the drain and therefore in preventing deterioration due to hot carrier injection. This type of problem becomes more tangible, the more the characteristics increase, and the higher the required functionality of the active matrix type liquid crystal display device, particularly in a crystalline silicon TFT. Therefore, considering the different TFT operating state, and considering the prevention of the above hot carrier effect, it is necessary to optimize parameters such as the impurity element concentration of the LDD region, and its distribution.